Red blood cell biomechanics in birdshot retinochoroidopathy using dual-trap optical tweezers technique – A possible hypothesis for non retinal vascular occlusion.

Abstract

Purpose : Birdshot retinochoroidopathy (BCR) is a severe chronic posterior uveitis without retinal vascular occlusion and no systemic involvement. The objective of this study was to investigate the red blood cell (RBC) biomechanics in patients with BCR using optical tweezers technique and to hypothesize the cause for non-occlusion in patients with BCR.

Methods : Ethics board approval was obtained. Bloods from age matched control and patients was collected for standard biochemical and hematological tests and for RBC deformability assessment using dual-beam optical tweezers. A dual-beam optical tweezers (two trapping beams) was made by splitting and recombining a single Nd: YAG laser beam. RBCs were trapped directly (i.e. without microbead handles) in the dual-beam optical tweezers (60% laser energy) where they adopt a "side-on" image. Initial length, final stretched length, difference between initial and final stretched length of RBC was calculated and Deformability index (DI) was computed. Mean with Standard deviation was calculated for all the variables. Spearman’s rho correlation coefficient with Bonferroni adjusted p values and regression analysis was performed to study the correlation between different variables and outcome.

Results : Blood from 8 healthy controls with mean age of 52.37 yrs and 9 patients with BCR with mean age of 53.44 yrs was analyzed (p=0.85). Unstretched average length of RBCs for control group was 8.48μm (±0.25, Range: 8.03-9.25) and 8.87μm (±0.31, Range: 8.18–9.73) for BCR RBCs (rho=0.027, p=0.27). The DI for control group was 0.066(±0.02, Range: 0.017-0.131) and that for BCR RBCs was 0.063 (±0.03, Range: 0.012-0.270) (rho=-0.026, p=0.28). None of the hematological factors in the blood seem to have an effect on initial length of RBC and DI on regression analysis.

Conclusions : DI of RBCs from BCR was statistically insignificant from normal healthy controls suggesting plausible hypothesis for non-occlusion of retinal veins despite significant inflammation. None of the biochemical or hematological factors were found to be influencing DI of the RBCs. Dual-beam optical tweezers method can be used as one of the tool to assess RBC deformability.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.

 

Initial red blood cell size and deformability index for patients with Birdshot retinochoroidopathy (green dots) and control group (red dots)